The invention relates to a capacitive humidity sensor.
A humidity sensor of this kind is a capacitor with at least two electrodes, between which a humidity-sensitive dielectric is located. At least one of the two electrodes, which can consist of a differently shaped metallic layer, is provided on an electrically highly insulating support which preferably consists of glass or ceramic and is frequently referred to as a substrate. The second electrode, outwardly located and likewise designed as a metallic layer, is permeable to moisture, especially to water vapor, i.e. the water molecules in the air can diffuse through it.
Between these two electrode layers is the humidity-sensitive dieletric, critical for humidity measurement.
In previously known humidity sensors of this kind, a polymer film is used as the dielectric layer.
The company known as Vaisala Oy, Helsinki, offers under the name of "Humicap" a thin-layer sensor in which a humidity-sensitive polymer of the cellulose acetate type is mounted on a glass substrate as a carrier for two-layer gold electrodes. In this arrangement, the second electrode is formed by a very thin gold electrode 100 to 200 angstroms thick, serving as a common reference electrode for the two electrodes mounted on the glass substrate. These two lower electrodes are contacted, so that the capacitance between the two lower electrodes is measured with the polymer dielectric, with the dielectric field lines running parallel to one another to the upper, zero-potential electrode.
In another humidity sensor made by the Coreci Company and designated "H 2000", a polymer is again used as the dielectric and consists of cellulose acetate butyrate. The lower electrode is made of tantalum sputtered onto a glass substrate, said tantalum then being oxidized. The outwardly located humidity-permeable electrode consists of a 1 micron-thick chromium layer, which is contacted by a chromium-nickel-gold electrode. To decrease the response time, this sensor arrangement is subjected after manufacture to a heat treatment such that the chrome electrodes, together with the polymer layer below them, are fractured to form trenches.
German 3,339,276 A1 teaches a similar capacitive humidity sensor, in which a polyimide is used as the humidity-sensitive layer. Comb-shaped gold electrodes are mounted on this dielectric layer, said electrodes being staggered with respect to the tantalum electrodes likewise arranged combwise on the glass substrate. This arrangement has the advantage that the water molecules can penetrate directly and hence very rapidly into the humidity-sensitive layer.
Use of a polyimide foil for a capacitive humidity sensor is also known from DE 28 48 034 A1. This humidity sensor has two metallic layers forming the electrodes, one of which is a thin gold layer permeable to water vapor while the other is made of a non-rusting iron-chromium-nickel steel.
The change in capacitance of a humidity sensor of this kind in the presence of air with different moisture content is based on the fact that the water molecules in the air diffuse into the polymer film forming the dielectric, changing the dielectric constant (DC) and thus the capacitance of the resultant capacitor. While the dielectric constant of polymers is between 2 and 3, the dielectric constant of water is 80. This means that when water molecules penetrate the dielectric layer, the capacitance of a capacitor that can be used to measure humidity, increases.
It is also important for measurement results to be both reproducible and largely independent of other physical or chemical factors.
The following properties are important for selecting the polymer layer for the dielectric:
1. Reproducible dependence of the capacitance on relative humidity (RH). PA0 2. Ideally no drifting, especially at high humidity levels. Previously known humidity sensors using polymer dielectrics do not yet meet this condition satisfactorily. PA0 3. Ability to withstand high temperatures. PA0 The majority of sensors known thus far can be used only at temperatures up to 80.degree. C. PA0 4. Usability over the entire humidity range from 0 to 100% RH. PA0 5. High resistance to foreign gases such as SO.sub.2, C.sub.x H.sub.y, NH.sub.3, etc.
Use of previously known humidity sensors at humidities above 90% RH is only possible for short periods of time as a rule, since in this range the electrical characteristics of the humidity sensor are no longer reproducible, in other words the capacitance of the sensors shows pronounced drifting.
The dielectrics used heretofore, especially polymers from the polyimide group, show pronounced drifting and a lack of reproducibility as regards electric characteristics when used in humidity sensors at humidity levels about 90% RH.